Sound attenuation is an important factor in the design of a car. For sound attenuation fibrous materials are used in mass spring acoustic systems as well as in single or multilayer absorbing systems.
The choice of a particular sound insulating material for a given application is determined not only by its ability to attenuate sound but by other considerations as well. These include cost, weight, thickness, fire resistance, etc. Well-known sound attenuating materials include felts, foams, compressed fibrous felt materials, glass wool or rock wool, and recycled fabrics including shoddy materials.
For instance U.S. Pat. No. 5,298,694 discloses an acoustical insulating web to be used as an absorbing layer, comprising melt-blown microfibers and crimped bulking fibres in a weight ratio of about 40:60 to about 95:5. The crimped bulking fibers disclosed are mechanically crimped fibers or thermally crimped fibers. These types of crimp are mainly used to aid the production process of the fibrous material felt layer, however they do not have a prolonged effect on the product performance during its use.
EP 934180 A discloses a multilayer acoustic trim part with at least two layers whereby the top layer is compressed to form a micro porous stiffening layer having a total airflow resistance of between Rt=500 Nsm−3 to Rt=2500 Nsm−3 and an area weight of between 0.3 kg/m2 and 2.0 kg/m2. The part showing sound absorption properties
For the parts described in this and similar patents as well as found in cars, the layers are normally formed together to obtain an overall multilayer construction. One way to produce a layer which is part of the multilayer is to distribute the fibers in such a way that the area weight (mass per unit area) of the layer remains constant. In this case, if the layers are put together on top of each other through the forming process, the overall area weight of the multilayer is still constant, while the overall density of the multilayer is varying from point to point. In particular, in areas where the layers are compressed to obtain a lower thickness on the part, the overall density is higher than in areas where the layers are less compressed to fill a space with higher thickness. For this reason and for this type of parts, high overall density of the multilayer is generally associated to low thickness and low overall density of the multilayer is generally associated to high thickness.
It is estimated that up to 30% of the total area of parts forming the state of the art is not contributing to the sound absorption of such parts due to local areas with high density at low thickness rendering the product near to impervious to air in those areas of the part.
The estimation of 30% weak areas comes from analysis of typical packaging space, i.e. available volume to be filled by acoustic parts in a vehicle. For such parts, the range of thicknesses is generally between 5 and 60 mm, but the distribution of thickness and the extreme values can vary between different cars and parts. For typical dash inner acoustic parts that are in majority of absorptive type, the thickness distribution found is roughly as follows: thickness distribution below 7.5 mm 19%, with a thickness distribution between 7.5 and 12.5 mm 27%, with a thickness distribution between 12.5 and 17.5 mm 16%, with a thickness distribution between 17.5 and 22.5 mm 13%, with a thickness distribution between 22.5 and 27.5 mm 20%, and with a thickness distribution above 27.5 mm 5%.
These data show that the thicknesses below 12.5 mm highly contribute to the overall area of the part (about 45%). In these areas, the material is heavily compressed and this has a negative impact on the acoustic performance, in particular for thicknesses below about 8 mm. The location of part of these low thickness areas is at the edges and around the cut-outs and therefore less important, however a good part of the 45% is strongly contributing to the performance. For these considerations, it is estimated that roughly 30% of the area of a typical part has characteristics which are especially critical for the overall performance.
Another important issue is that currently used fibrous material is unable to achieve sufficient thickness at low density to address part thickness requirements. Therefore weight is added to obtain the required thickness, however at the cost of the increasing overall weight of the part. Adding weight has in turn a negative effect on the acoustic performance of the lower thickness areas where the material is heavily compressed. Not only the packaging space available is relatively limited and influencing the performance of the part, but in addition the increase of weight limits even more the performance in these areas. Overall, due to the material currently used and the problem just described, approximately up to 30% of the area is minimally or not contributing to its overall acoustic performance.
It is therefore the object of the current invention to further optimise the multilayer absorbing products of the state of the art, in particular to further optimise the overall acoustic performance of the part.